The invention relates to a process for making thick fibrous structures. More particularly, the invention relates to a simplified needle-felting process wherein fibrous material is accreted into a thick fibrous structure by repeatedly driving a multitude of felting needles into the fibrous material, the felting needles penetrating all the way through the fibrous material at the beginning of the process, and penetrating only part way through the fibrous material at the end of the process.
According to certain processes known in the art for making fibrous structures, several coherent fibrous layers are needled together in a series of needling steps wherein a multitude of felting needles are repeatedly driven into the fibrous layers. The felting needles are forked or barbed and engage fiber from the layers as they are driven into the layers, and permanently transport bundles of fibers between layers, the bundles of fibers cohering the individual layers into a coherent fibrous structure. In certain processes for making thick fibrous structures, the felting needles penetrate all the way through the fibrous layers disposed beneath the felting needles at the beginning of the process, and penetrate only part way through the fibrous layers disposed beneath the felting needles at the end of the process. Up to twenty, thirty, or more layers may be cohered into a fibrous structure. In such processes, each individual layer is typically previously prepared into a coherent layer in sheet form in the shape of a rectangle, annulus, or sector of an annulus. The coherent layers in sheet form may be made from various coherent fibrous materials, including woven fiber, knitted fiber, braided fiber, and previously felted fiber. Examples of typical processes for making thick fibrous structures in such manner are provided by U.S. Pat. Nos. 3,657,061, 4,790,052, and 4,955,123. The intermediary process of making the individual coherent layers adds additional cost and complication to the overall process, but using only coherent layers to form a thick fibrous structure in the manner described was previously thought necessary to enable build-up of a thick fibrous structure, and in order to provide a thick fibrous structure having suitable physical characteristics.
An object of the invention is to provide a simplified process for making thick fibrous structures without affecting the ability of the process to form such structures, and without compromising physical characteristics. Processes according to various aspects of the invention are described herein that fulfill at least these objectives. Such processes are greatly simplified relative to prior art processes because loose fiber is directly accreted into a thick fibrous structure, without a prior step of cohering the fiber into individual coherent layers.